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A remote sensing platform--the vehicle or structure used to carry a sensor--is designed with a relatively narrow set of purposes in mind. Many important decisions must be made when designing a remote sensing technology. The type of sensor and its capabilities must be defined. The platform on which the sensors will be mounted must be determined. The means by which the remotely-sensed data is received, transmitted, and processed before delivery to its end user must be designed. All of these decisions are made based on knowledge of the target and the information about the target that is in demand, balanced by other factors such as cost, availability of resources, and time constraints. The end result of this process is a tool that is specifically designed to perform a task or a set of related tasks that will assist researchers in better understanding the process that is under investigation.

Sensor Resolution

There are many applications of remote sensing, and each sensor is engineered for very specific purposes. The design and placement of a sensor is determined by the unique characteristics of the target that will be studied and the information that is required from the target. Each remote sensing application has specific demands on the amount of area to be covered, the frequency with which measurements will be made, and the type of energy that will be detected. Thus, a sensor must provide the spatial, spectral, and temporal resolution necessary to meet the needs of the application.

Spatial resolution refers to the amount of detail that can be detected by a sensor. Detailed mapping of land use practices requires a much greater spatial resolution than observations of a large scale storm system. Thus, satellites such as Landsat generally have greater spatial resolution than global weather satellites.

Spectral resolution refers to the width or range of each spectral band measured by a sensor. A spectral band is the wavelength range recorded by a sensor. Detection of some phenomena, such as vegetative stress, requires a sensor with sensitivity in a narrow spectral band so that differences in the spectral signatures at a specific wavelength can be detected. A panchromatic sensor, which covers a wide spectral range, would not be well suited to such a task. A narrow band sensor in the red portion of the spectrum would be better at detecting vegetative stress.

Temporal resolution refers to the time interval between measurements. For some applications, such as monitoring the development of a severe thunderstorm, measurements are required at a frequency of a few minutes. Some applications, such as measuring crop production or insect infestations, require seasonal measurements, while others, such as geological mapping, require a single measurement.

Ground-Based Platforms

Remote sensing platforms that position the sensor at the Earth's surface are called ground-based platforms. These systems are fixed to the Earth and the sensors are often standard tools used to measure environmental conditions such as air temperature, wind characteristics, water salinity, earthquake intensity and such. Ground-based sensors can be placed on tall structures such as towers, scaffolding, or buildings to elevate the platform.

Ground-based sensors are generally less expensive to operate and maintain than aircraft or satellite sensors, but they do not provide the aerial extent of the airborne platforms. Ground-based sensors are often used to record detailed information about the surface, which is compared with information collected from aircraft or satellite sensors.

One example of ground-based remote sensing are sensors mounted on buoys that make real-time measurements of water temperature, salinity, wind speed, and wind direction. The buoys are anchored in a body of water (the target) and they transmit the results of each measurement to receiving stations to be processed. These sensors can be used to supplement or "ground truth" measurements made from airborne or satellite sensors.

Aerial Platforms

Aerial platforms are most often sensors mounted on fixed-wing aircraft, though other airborne platforms, such as balloons, rockets, and helicopters can be used. Aircraft are often used to collect very detailed images of the Earth's surface and facilitate the collection of data over virtually any portion of the Earth's surface at any time. Aerial systems elevate the sensor above the Earth's surface in order to increase its aerial coverage. They also allow researchers to monitor very large areas of the surface which would be impractical with ground-based sensors or impossible or dangerous to visit.

Airborne remote sensing dates back to the early 1900's when airplanes were used during the World Wars to conduct surveillance of the enemy. More recently, cameras mounted on aircraft have been used to monitor land use practices, locate forest fires, and produce detailed and accurate maps of remote or inaccessible locations on our planet. Weather balloons and rockets are still used by research scientists as a means for obtaining direct measurements of the properties of the upper atmosphere. These provide a less expensive and reusable alternative to aircraft and satellite systems.

The following images depict Atlanta, Georgia as seen from an airborne sensor mounted on a specially equipped Learjet. These images were part of a study on the effect of urban sprawl on the temperature within a city.

True Color Image of Atlanta, Georgia
Thermal Infrared Image of Atlanta, Georgia
Figure 1. True Color and Thermal Infrared Images of Atlanta, Georgia
(from NASA Goddard Space Flight Center Scientific Visualization Studio)

In the early 1960's researchers started mounting sensors on satellites placed into orbit over the Earth and ushered in a new era of environmental remote sensing that continues to grow at a rapid pace today. The vantage point of space allows researchers to observe and measure phenomena on a time and spatial scale that was previously impossible. Today, satellites provide us with views of the Earth that allow us to monitor global change and understand our planet.

This wealth of data comes with a price, however. To build a satellite and place it into orbit is a very difficult and expensive endeavor, often coming with a price tag that approaches billions of dollars. Satellites must be operated remotely from the ground and data from the satellite sensors must be transmitted to the surface. The communications technologies in remote sensing satellites can be very complex and expensive to engineer and maintain. A number of satellites have failed to reach orbit, or failed to operate once in orbit around the earth, which is a testament to the incredible complexity involved in designing, building, and operating a satellite.

All of these difficulties not withstanding, environmental satellites have contributed greatly to our understanding of the Earth's environment and continue to be used extensively for remote sensing research. For example, weather satellite technology, one of the first practical applications of satellite remote sensing, has vastly expanded our understanding of the Earth's weather by providing a synoptic (large scale) view of our weather systems that was previously impossible. It was only after the advent of satellites that weather patterns such as hurricanes and mid-latitude cyclones were fully understood. Prior to satellites, any knowledge of these storms was collected through ground level observations that unfortunately did not provide the information necessary to adequately understand them. The contribution of satellites to our understanding of dangerous weather events has saved countless numbers of lives since the early 1960's.

Landsat-Derived Record of Increasing Urbanization Through Time Along the Potomac River, Virginia and Maryland
Figure 2. Hurricane Isabel, September 18, 2003

Satellites such as those in the Landsat program have monitored land use change for decades, providing detailed insight into how development has affected tropical rain forests, how climatic changes have affected agricultural production, how deserts advance and withdraw, and how the polar ice caps have retreated. The Landsat program, which started in 1972 with the launch of the Landsat 1 satellite, continues even today with Landsat 7, which provides us with daily images of parts of the Earth's surface.

Landsat-Derived Record of Increasing Urbanization Through Time Along the Potomac River, Virginia and Maryland
Figure 3. Landsat-Derived Record of Increasing Urbanization Through Time
Along the Potomac River, Virginia and Maryland

Communications and Data Collection

Data collected from a remote sensing system must be retrieved and delivered to the end users. Often, this must be done quickly for the data to be of any use, such as in the case of severe thunderstorm forecasting where storms develop into severe storms within minutes. Thus the transmission, reception, processing, and distribution of data from a satellite sensor must be carefully designed to meet the users' needs.

Ground-based remote sensing platforms can transmit data using ground-based communication systems, such as radio and microwave transmissions or computer networks. Some systems can store data on the platform, allowing researchers to manually collect the data from the platform. Data collected in an aircraft can be stored on board and retrieved once the aircraft lands. Satellite data, however, is very difficult to obtain since the satellite remains in space during its entire operational lifetime. This data must be transmitted back to the Earth to a ground receiving station, which can receive the data and process it for distribution to the end user.

Data collected from a satellite platform can be transmitted to Earth in a variety of ways. A satellite can transmit data directly to a ground receiving station that is within its line of sight. When the satellite is not in sight of a ground station, it can store its data on board and "dump" the data later, when it is back in sight of a ground station. Finally, for immediate transmission, a satellite can relay data to the ground receiving station through a series of communications satellites in orbit around the Earth, transferring data from one satellite to the next until it is able to reach the ground receiving station desired.

Many satellites use a combination of the methods described above. The TIROS class meteorological satellites (NOAA satellites) use a continuous transmission of lower resolution data that can be received from any ground station within radio range while also using on-board storage to store higher resolution data that is transmitted to specific ground stations capable of receiving it.

The data received at the ground station are in a raw digital format. They may then, if required, be processed to correct systematic, geometric and atmospheric distortions to the imagery, and be translated into a standardized format. The data are written to some form of storage medium such as tape, disk, or CD. The data are typically archived at most receiving and processing stations, and full libraries of data are managed by government agencies as well as commercial companies responsible for each sensor's archives.


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